DNA interstrand crosslinks (ICLs) present a formidable block to DNA metabolic processes and must be repaired for cell survival. While much work has been done to define the mechanisms of ICL repair in bacteria and yeast, their processing in mammalian cells is not clearly defined. In the previous funding period, we demonstrated an nucleotide excision repair (NER)-dependent, error-prone repair of triplex-directed psoralen ICLs in mammalian cells. We also found that mismatch repair (MMR) proteins are involved in the response to and repair of psoralen ICLs in an error-free process. Our working hypothesis in this renewal application is that NER and mismatch repair (MMR) proteins interact in the recognition and initial processing of ICLs, while MMR proteins, independently of NER factors, are involved in ICL-induced cell-cycle regulation and apoptosis. The long-term objectives of the proposed research are to elucidate molecular mechanisms involved in the removal of DNA ICLs from the mammalian genome, to identify interactions among proteins from HR, MMR, and NER pathways in doing so, and to determine the roles of these proteins in other cellular responses to these lesions. Specifically we propose to: 1) test the hypothesis that proteins from the NER and MMR pathways interact during recognition and initial processing of ICLs;2) test the proposal that MLH1 functions in cellular checkpoint and apoptosis responses to DNA ICLs;3) target DNA ICLs to specific genomic sites in mutant mammalian cell lines to determine roles for DNA repair and recombination gene products in processing ICLs;and 4) assess the potential of targeted ICLs as antiproliferative therapeutic agents. The results obtained from these studies will provide valuable information to develop improved targeted strategies to control human cancers using ICL-inducing agents.